SMH4811A
Preliminary
APPLICATIONS
Operating at High Voltages
IDMAX × VSMIN
IDMIN
Substituting:
IDMIN
=
=
Thebreakdownvoltageoftheexternalactiveandpassive
components limits the maximum operating voltage of the
SMH4811A hot-swap controller. Components that must
be able to withstand the full supply voltage are: the input
and output decoupling capacitors, the protection diode in
series with the DRAIN SENSE pin, the power MOSFET
switch and the capacitor connected between its drain and
gate, the high-voltage transistors connected to the power
good output, and the dropper resistor connected to the
controller’s VDD pin.
.
VSMAX
250µA × 36V
= 125µA
.
72V
Now the value of R3 is calculated from IDMIN
:
VSMIN − V
UV
R3 =
.
IDMIN
Over-Voltage and Under-Voltage Resistors
V
UV istheunder-voltagetrippoint,also2.5V. Substituting:
In the following examples the three resistors, R1, R2, and
R3, connected to the OV and UV inputs must be capable
of withstanding the maximum supply voltage of several
hundred volts. The trip voltage of the UV and OV inputs is
2.5VrelativetoVSS. AstheinputimpedanceofUVandOV
is very high, large value resistors can be used in the
resistive divider. The divider resistors should be high
stability,1%metal-filmresistorstokeeptheunder-voltage
and over-voltage trip points accurate.
36V − 2.5V
125µA
R3 =
= 268kΩ
.
The closest standard 1% resistor value is 267kΩ
Then R2 is calculated:
VUV
R1+R2 =
(
)
,
IDMIN
Telecom Design Example
A hot-swap telecom application may use a 48V power
supplywitha–25%to+50%tolerance(i.e.,the48Vsupply
can vary from 36V to 72V). The formulae for calculating
R1, R2, and R3 follow.
or
V
UV
R2 =
−R1
.
IDMIN
First a peak current, IDMAX, must be specified for the
resistivenetwork. Thevalueofthecurrentisarbitrary, but
it can't be too high (self-heating in R3 will become a
problem), or too low (the value of R3 becomes very large,
and leakage currents can reduce the accuracy of the OV
andUVtrippoints). ThevalueofIDMAX shouldbe≥200µA
forthebestaccuracyattheOVandUVtrippoints. Avalue
of 250µA for IDMAX will be used to illustrate the following
calculations.
Substituting:
2.5V
R2 =
−10kΩ = 20kΩ −10kΩ = 10kΩ
.
125µA
An Excel spread sheet is available on Summit's website
(www.summitmicro.com) to simplify the resistor value
calculations and tolerance analysis for R1, R2, and R3.
Dropper Resistor Selection
With VOV (2.5V) being the over-voltage trip point, R1 is
calculated by the formula:
The SMH4811A is powered from the high-voltage supply
via a dropper resistor, RD. The dropper resistor must
provide the SMH4811A (and its loads) with sufficient
operating current under minimum supply voltage condi-
tions,butmustnotallowthemaximumsupplycurrenttobe
exceeded under maximum supply voltage conditions.
VOV
R1 =
.
IDMAX
Substituting:
The dropper resistor value is calculated from:
2.5V
R1 =
= 10kΩ
.
250µA
VSMIN − V
DDMAX
RD =
,
IDD +ILOAD
Next the minimum current that flows through the resistive
divider, IDMIN, is calculated from the ratio of minimum and
maximum supply voltage levels:
SUMMIT MICROELECTRONICS, Inc.
2044 6.1 2/8/01
10
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
